The present invention relates to high volume low pressure air spray guns, and in particular to a hydraulically assisted air spray gun in which high volume low pressure air pneumatically atomizes liquid coating material that is hydraulically emitted in a fan-shaped pattern from an airless tip.
To decrease the cost of coating material used in spray coating processes and for environmental considerations, there has been a trend toward spray coating equipment having a high transfer efficiency. Transfer efficiency is the amount of coating solids applied onto a target versus the amount of coating solids sprayed, expressed as a percentage. To increase transfer efficiency, the velocity of the coating particles should advantageously be fairly slow in order to avoid blow-by which occurs when spray particles miss the target, with excessive velocity of the particles actually causing some of the particles that strike the target to bounce off of it. Greatest transfer efficiency is usually achieved in systems offering optimum atomization coupled with the lowest possible velocity of the particles.
Conventional air spray guns have a relatively low transfer efficiency. Air delivered to their spray heads has relatively high pressure on the order of 25 psi or more and as it exits the spray head it atomizes a cylindrical stream of liquid coating material into a conically-shaped spray, which usually is flattened into a fan-shaped pattern by opposed side port air jets. When the high pressure air exits the spray head, it expands and imparts a relatively high velocity and fogging effect to the spray particles, causing a large percentage of the particles to miss the target.
Airless spray systems have a somewhat higher transfer efficiency. With such systems, coating liquid is hydraulically forced through a specially shaped orifice at pressures on the order of 500-4500 psi, which causes the coating to be emitted in an unstable thin film that breaks up into an atomized spray at its forward edge. These systems develop spray particles that have a lower velocity and exhibit less fogging than occurs with conventional air spray guns.
A more recent development is the air-assisted airless system, which utilizes both airless and air atomization. Coating liquid is supplied to a specially shaped orifice at hydraulic pressures less than those normally encountered in purely airless systems, usually on the order of 300-1000 psi. This causes the material to be atomized into a spray, but the degree of atomization is not as satisfactory as is obtained with conventional airless or air spray guns. To improve atomization, an air assist is applied to the spray pattern, enhancing the atomization process and doing away with tails that would otherwise mar the finish. The transfer efficiency of air-assisted airless systems is greater than those of conventional airless or air spray systems.
Recently, high volume low pressure (HVLP) air spray systems have found increasing use because of their high transfer efficiency. These systems rely solely upon pneumatic atomization and utilize air to atomize a stream of coating material. At the spray head the air has a relatively high volume flow rate, usually well in excess of 5 CFM, and a relatively low delivery pressure, usually less than 15 psi. The high volume and low pressure of the air results in decreased fogging and an increased percentage of the spray particles striking and adhering to the target.
Some HVLP spray guns use a turbine to supply air at high volume and low pressure to an inlet to the gun, from which it passes through enlarged air passages to the spray head. A significant disadvantage is that a separate turbine is required for supplying air, which increases the cost and complexity of the system. Other HVLP spray guns, such as the one disclosed in U.S. Pat. No. 3,796,376 to Farnsteiner, receive high pressure factory air at their inlet. Such guns have a venturi in their handle air passage downstream from the inlet, to reduce the pressure and increase the volume flow of air into the gun body. To further increase the volume flow of air into the gun, in the spray gun of U.S. Pat. No. 3,796,376, passages in the handle admit atmospheric air by the action of the compressed air passing through the venturi. From the venturi, air then passes at a reduced pressure and increased volume through passages in the gun body to the spray head. Another HVLP spray gun is disclosed in U.S. Pat. No. 4,761,299 to Hufstetler.
It is desirable with HVLP spray guns to be able to control the shape of the spray pattern. Conventionally, this requires that the cylindrical stream of coating material that is broken up into a conically diverging atomized spray be selectively shaped between conical and flat fan by directing jets of side port air against opposite sides of the spray. However, with many such guns no provision is made to control the pressure of air at the spray head as the flow of side port air is varied. In consequence, an undesirable increase in spray head air pressure can occur when the side port air flow rate is reduced.
Although some prior HVLP spray guns, such as the one of said U.S. Pat. No. 4,761,299, develop at the spray head a relatively low pressure of air on the order of 15 psi or less, it has become desirable to limit the maximum pressure of air at the spray head to 10 psi or less. This is because HVLP spray guns that are limited to an air pressure of 10 psi or less at the spray head inherently have a high transfer efficiency. As a result, certain environmental protection agencies, such as those in California, which otherwise would require as a condition for use of a spray gun that it be tested to meet at least a specified minimum transfer efficiency, automatically exempt a gun from testing if the pressure of air at its spray head is 10 psi or less.